Main Entry: linch·pin

Variant(s): also lynch·pin \ˈlinch-ˌpin\

Function: noun

Etymology: Middle English lynspin, from lyns linchpin (from Old English lynis) + pin; akin to Middle High German luns linchpin

Date: 13th century

1 : a locking pin inserted crosswise (as through the end of an axle or shaft)
2 : one that serves to hold together parts or elements that exist or function as a unit <the linchpin in the defense's case>